Synthetic glycoconjugates as experimental vaccines E. coli O148 is a human enteropathogen that occasionally causes dysentery, a serious gastrointestinal disease in both developed and developing countries, similar to that caused by S. dysenteriae type 1. The O-specific polysaccharide (O-SP) of E. coli O148 is similar to that of S. dysenteriae type 1. Both are composed of tetrasaccharide repeating units, the only difference between them is replacement of the glucose residue in the former by a galactose residue in the latter. The component monosaccharides are connected at the same positions in the identical anomeric stereochemistry in both. This similarity, promptedus to investigate O-SP cross-reactivity and potential cross protection afforded be antibodies to one to infection with the other. To this end we have synthesized chemically saccharide fragments ranging from mono- to dodecasaccharides. The saccharides were equipped with a linker that enabled their covalent attachment to proteins using our conjugation protocol, based on stable oxime formation between the saccharides and the proteins. We have found that antisera to the oligosaccharide-protein conjugates reacted not only with the homologous O-specific polysaccharides but also with the heterologous ones. This finding establishes cross-reactivity in both directions and supports the view that an O-specific oligosaccharide-based vaccine against one of the organisms will offer protection against both bacteria. Borrelia burgdorferi is the causative agent of Lyme disease that afflicts a large number of people of all ages, including children, in large areas of the US, prominently in the northeastern part of the country. No vaccine against this organism is licensed. B. burgdorferi expresses two groups of glycolipids on its surface termed BBGL-1 and BBGL-2. We surmised that BBGL antibodies would offer protection against this bacterium. Following earlier work on BBGL-1, our attention is turned to BBGL-2. This fraction is composed of alpha-galactosyl-glycerol that bears two fatty acid moieties on the glycerol part. In order to map the importance of the various structural moieties for antibody induction, we have synthesized chemically a panel of BBGL-2 analogs that contain the galactosyl moiety in either alpha or beta anomeric configuration, and have the fatty acid components in alternate positions. We found that the anomeric configuration of the galactose moiety is not important in antibody recognition. We also found that the glycerol moiety has to bear two fatty acid moieties of which one must be an oleoyl moiety that may be at either sn-position 1 or 2. Based on these findings we synthesized conjugatable derivatives of the native BBGL-2 glycolipid and attached them to BSA. The synthetic glycolipoprotein induced specific antibodies in mice. This observation forms the basis for the synthesis of our next generation BBGL-2 conjugates. Haemophilus influenzae type a (Hia) is a human pathogen causing meningitis especially in children. Similar to Haemophilus influenzae type b, it has been proposed that a critical level of antibodies against the capsule of Hia can prevent Hia infection. The capsule of Hia is composed of repeating units of glucosyl-ribitol-phosphate. Based on published evidence, we believe that oligomers of the repeating unit may induce anticapsular antibodies when covalently linked to an immunogenic protein carrier. In order to test this hypothesis, we designed a chemical synthetic approach to the capsular polysaccharide of Hia based onchemical synthesis of a complete repeating unit followed by its polymerization. Accordingly, we assembled a repeating unit linked to a spacer for eventual coupling to a protein. Next, we extended the polymers chain using another repeating unit. Iterative chain exension led to a trimer of the glucosyl-ribitol-phosphate rrepeating unit. We are currently optimizing the synthetic steps in order to allow the preparation of higher-membered oligomers.